A well known method to improve the linearity of a radio frequency amplifier is to use a Cartesian feedback loop. Referring to FIG. 1, a conventional Cartesian feedback loop 100 is shown that includes a baseband section 102, an RF section 106, and a modulator 104. FIG. 1 shows a differential half circuit with each line representing a differential signal, which is actually carried on two wires. Cartesian feedback loop 100 further includes two feedback loops in the baseband section 102—an “I-loop” for an in-phase signal and a “Q-loop” for a quadrature signal.
Each feedback loop includes an input amplifier 108 to amplify an input signal I, Q. Each feedback loop further includes a signal combiner 110 that can combine the amplified input signal and a baseband feedback signal 109 to produce a combined forward-path signal 111. The combined forward-path signal 111 in each feedback loop can be amplified by a forward-path amplifier 112. Each feedback loop further includes a rotator 118 to compensate for the delay in the RF section and to keep the I-loop and the Q-loop uncoupled. Each feedback loop further includes a loop filter 114 to filter the combined forward-path signal. Each feedback loop further includes a feedforward mixer 116 to modulate the combined forward-path signal and produce an RF signal.
Cartesian feedback loop 100 further includes a feedback mixer 126 to demodulate the RF signal and produce a baseband feedback signal. Cartesian feeback loop 100 further includes a feedback baseband amplifier 128 to amplify the resulting baseband feedback signal.
A well known problem with a Cartesian feedback loop, such as the one shown in FIG. 1, is the presence of a phase shift in the baseband section due to parasitics associated with integrated circuit components. A large phase shift in a Cartesian feedback loop can result in peaking and/or oscillation.